Could it be that SP is using this:
[from http://www.epanorama.net/circuits/microphone_powering.html ]
Simplest circuit-- This circuit is a very simple method of connecting
an electret microphone capsule to the a balanced phantom powered
(48V) mixer XLR input:
(illustration)
http://ad2004.hku.nl/naturesound/RobD/ElectretOnPhantomHack.jpg
Note that this is a very simple "hack" to interface an electret
microphone capsule to a mixer. This circuit works but has it's
downsides like sensitivity to noise in phantom power, unblanced
signal transmission (prone to interference) and high output impedance
(can't properly drive long cables). This circuit can be used to test
electret capsules connected to mixer using very short cable. This
circuit makes very high level popping noise when it is
connected/disconnected from the mixer or when the phantom power is
turned or disconnected. Other downside is that this circuit loads the
phantom power in very unbalanced way which can disturb some older
mixers (in some mixers input transformer can saturate if such exists,
in that case try adding 47 ohom resistor between pins 1 and 3).
In practice this circuit seems at least to work on modern mixers, but
I don't recommend this circuit for any real recording or PA
application For any real use you are much happier with a better
quality balanced circuit. They are more complicated but perform much
better.
Rob D.
=3D =3D =3D =3D
At 6:04 PM -0700 5/3/08, Aaron Ximm wrote:
>Well,
>
>Motivated by several requests for follow-up on this in the past several
>days, I put down my 12-week-old daughter for a while today, and tried
>various combinations of plug-in powered mics, my Sound Devices 722, and th=
is
>cable:
>
><http://www.soundprofessionals.com/cgi-bin/gold/item/SP-XLRM-MINI-2>http:/=
/www.soundprofessionals.com/cgi-bin/gold/item/SP-XLRM-MINI-2
>
>It looked very promising, being not only a 1/8" stereo miniplug to dual XL=
R
>male adapter, but also a phantom power to plug-in power adapter as well.
>With no "box," just a normal looking cable, I had high hopes that this was
>simplify my kit when using mics that require plug-in power.
>
>Sadly, my results so far are not encouraging -- though there is a caveat,
>which is that what I am hearing may result in most part from the Sound
>Devices front end's behavior when it is presented with what I assume is no=
t
>a balanced input while it is in "Mic" mode. I have reason to suspect this
>(though that doesn't help me in my case).
>
>I tried the test cable with a pair of Microphone Madness phantom-powered
>cardioid mics on a gooseneck, which I believe are based on Sennheiser MKE
>capsules; a pair of the venerable Radio Shack 33-3028 stereo lav mics; and
>my go-to mics, my Sonic Studios DSM-6S/EHs, with and without the expensive
>Sonic Studios matched preamp in series.
>
>I traded some email with the SP folks when ordering, and they said that it
>should present about 9-10V to the mics as I recall. Being familiar with th=
e
>warning of Leonard Lombardo (Sonic Studio) on what his mics require in a
>Plug-in Power source -- "high current capacity but not high voltages" -- I
>also confirmed that the adapter would be able to supply said current. (I
>also tested my less dear mics with the cable first, lest their be any risk
>that it passed too high a voltage!)
>
>Both the MM goosenecks mics and the Sonic Studios mics (direct) had poor
>results when powered through the cable -- at least, into the input on the
>Sound Devices when set on "microphone." There was a distracting degree of
>noise that I am not used to, and what I can best describe as a "fluttery"
>couple-hertz irregular mid-low frequency beating. I would almost call it
>"stuttering" and though I don't think it's related to the use of this term
>for mics tapping out the MP-2, I in my own tests described it as "motor
>boating." In addition, my beloved DSMs in particular did not seem to be
>performing to the standard I am use to; their output was quite low. Combin=
ed
>with the high noise I would call the combination unusable.
>
>The 33-3028s worked better out of the gate, but at high gain I could still
>detect the same "fluttering" happening; for some reason though they did
>perform better than either of the other pairs of mics -- enough that a
>casual use might not have revealed the fluttering right away. Perhaps they
>are the "happiest" with the high voltage / low current (?) provided by the
>cable?
>
>What makes me suspect that the problem is partly a result of the Sound
>Devices' input [topology] is that the noise I am hearing is quite similar =
to
>the large amount of noise you get if you turn on the microphone inputs wit=
h
>no mic [load] attached -- I would wager in fact that what I am hearing is
>the *same* noise. I'm not an EE but I immediately suspected that the probl=
em
>may be that the SD is really expecting a balanced input and or the correct
>impedance?
>
>I actually have encountered a similar issue before, using a Sound
>Professionals dedicated plug-in power box to power my electret Aquarian H2
>hydrophone, and then breaking out to XLR into the 722... lots of noise. To=
o
>much to use this combination.
>
>Sometime soon I may repeat this test using my venerable Mackie 1202 VLZ as
>the phantom source -- just to see if its presumably quite different inputs
>work with the cable any better.
>
>As a sanity check, I also powered my DSMs through their dedicated Sonic
>Studios preamp, which both (perfectly) power and amplifies them, and used =
a
>standard break out cable to dual XLR to connect them to the 722 when in
>"Line" input mode. This case -- which I have used before -- works well; th=
e
>only limitation being that only 18 db of (additional) gain are available
>from the 722 -- I would appreciate a bit more, even on top of the gain
>provided by the Sonic Studios pre. In this configuration the limiting nois=
e
>floor seems to definitely be the mics, regardless, though; I get much, muc=
h
>better results than with the tested cable.
>
>Last fall I also used this combination -- DSMs -> Sonic Studios pre ->
>standard breakout cable -> 722 -- into the 722's input in *microphone* mod=
e;
>I encountered what I now believe based on these experiments is possibly th=
e
>same issue. I should look in the 722 manual to see if flipping that switch=
,
>in addition to offering more gain, changes the impedance on the input as I
>suspect...
>
>My goal btw was to use my DSMs directly with the 722, without the
>intermediate pre. The latter is very small -- think, a pager -- but it's o=
ne
>more box to wrangle, and I do sometimes forget to shut it off and hence bu=
rn
>down the (very long lasting) AA in it.
>
>I can say that for the moment, though, that I will continue to use the Son=
ic
>Studios sanctified powering options.
>
>Question for the EEs in the group ~ is it possible that there is some simp=
le
>modification to the cable that might alleviate what I am hearing? Or would
>we need to dissect the cable to see what's really going on and why the 722
>is unimpressed with this as in input? I know where we can get one. :)
>
>best,
>aaron
>
>--
><aaron.ximm%40gmail.com>
>quietamerican.org
>oneminutevacation.org
>
>83% happy
>9% disgusted
>6% fearful
>2% angry
>
>
>
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